feat(mappings): do not simplify the rhs of a mapping

runtime/doc/vim_diff.txt

@@ -366,8 +366,9 @@ Macro/|recording| behavior
   the results of keys from 'keymap'.
 
 Mappings:
-  Creating a mapping for a simplifiable key (e.g. <C-I>) doesn't replace an
+- Creating a mapping for a simplifiable key (e.g. <C-I>) doesn't replace an
   existing mapping for its simplified form (e.g. <Tab>).
+- The rhs of a mapping is not simplified when it is defined.
 
 Motion:
   The |jumplist| avoids useless/phantom jumps.

src/nvim/getchar.c

@@ -2885,8 +2885,8 @@ void set_maparg_lhs_rhs(const char_u *const orig_lhs, const size_t orig_lhs_len,
       mapargs->rhs_len = 0;
       mapargs->rhs_is_noop = true;
     } else {
-      replaced = replace_termcodes(orig_rhs, orig_rhs_len, &rhs_buf, REPTERM_DO_LT, NULL,
+      replaced = replace_termcodes(orig_rhs, orig_rhs_len, &rhs_buf,
-                                   cpo_flags);
+                                   REPTERM_DO_LT | REPTERM_NO_SIMPLIFY, NULL, cpo_flags);
       mapargs->rhs_len = STRLEN(replaced);
       mapargs->rhs_is_noop = false;
       mapargs->rhs = xcalloc(mapargs->rhs_len + 1, sizeof(char_u));

test/functional/ui/input_spec.lua

@@ -313,6 +313,11 @@ it('unsimplified mapping works when there was a partial match vim-patch:8.2.4504
   expect('xb')
 end)
 
+it('rhs of a mapping is not simplified', function()
+  command('nnoremap <Plug>foo <C-J>')
+  eq('<C-J>', funcs.maparg('<Plug>foo'))
+end)
+
 describe('input non-printable chars', function()
   after_each(function()
     os.remove('Xtest-overwrite')